Proportion Calculator
Solve proportions with missing values.
Input
Result
What is a Proportion Calculator?
A proportion calculator solves for a missing term in an equal ratio relationship. It uses cross-multiplication on the proportion a/b = c/d to find the unknown value instantly.
How to calculate proportions manually (the formula)
For a proportion a/b = c/d, cross-multiply: ad = bc. If solving for d, the formula is d = bc/a. Always check that your result makes the two ratios equal.
Example calculation
Solve 3/4 = 6/d. Cross-multiply: 3d = 6 × 4 = 24. Divide by 3: d = 8. Check: 3/4 = 6/8, both simplify to 0.75 ✓
Common mistakes
- Cross-multiplying incorrectly — Multiply the numerator of one ratio by the denominator of the other. Do not multiply both numerators or both denominators.
- Forgetting to check units — Ensure corresponding terms use the same unit of measurement for a valid proportion.
- Ignoring zero denominators — A zero in the denominator makes the fraction undefined. Check inputs before solving.
Frequently asked questions
What is a proportion?
A proportion is an equation that states two ratios are equal, written as a/b = c/d. It shows that two fractions represent the same relationship.
How do you solve a proportion?
Use cross-multiplication: multiply a by d and b by c, then solve for the missing variable. For a/b = c/d, if d is unknown, d = bc/a.
What is the cross-multiplication method?
Cross-multiplication means multiplying the numerator of one fraction by the denominator of the other and setting the results equal. It is the standard way to solve proportions.
What happens if one term is zero?
Zero in the denominator makes the fraction undefined. If the numerator is zero, the proportion can still be solved but with care.
Can proportions be used with percentages?
Yes. A percentage is a proportion where the second ratio is out of 100. For example, 25% = 25/100 = 1/4.
What is the difference between a ratio and a proportion?
A ratio compares two quantities (e.g., 2:3). A proportion states that two ratios are equal (e.g., 2:3 = 4:6).